Filled polypropylene articles have been observed to exhibit undesirable odors, particularly upon heating. In this respect, see U.S. Pat. No. 5,023,286 to Abe et al., wherein phenolic antioxidants are suggested to control the odor problem. Other polypropylene compositions may be found in U.S. Pat. No. 4,734,450 to Kawai et al.; U.S. Pat. No. 5,045,369 to Kobayashi et al.; U.S. Pat. No. 5,300,747 of Simon; U.S. Pat. No. 5,439,628 of Huang and U.S. Pat. No. 4,933,526 of Fisher et al.
This invention relates to disposable, polypropylene/mica microwaveable containers having suitable food contact compatible olfactory properties including cups, trays, soufflxc3xa9 dishes, lids, plates, bowls, and related articles of manufacture useful for preparation, storage, delivery, and serving of food, wherein convenience and low cost are of paramount importance. Nevertheless, suitable food contact compatible olfactory properties, appearance, and tactile characteristics of the plate, container, etc., are important for consumer preference. The suitability of these disposable articles of manufacture for microwave cooking, or heating of food, has an important place in today""s marketplace. Both the commercial and retail market components need an aesthetically pleasing microwaveable, disposable, rigid and strong container, plate, or cup, and related articles of manufacture which also have suitable food contact compatible olfactory properties.
These disposable microwaveable containers and plates exhibit a melting point of no less than about 250xc2x0 F., the containers or plates being dimensionally stable and resistant to grease, sugar and water at temperatures up to at least 220xc2x0 F. and exhibiting sufficient toughness to be resistant to cutting by serrated polystyrene flatware and also exhibiting food contact compatible olfactory properties. The preferred containers and plates exhibit both suitable food contact compatible olfactory properties and at least one micronodular surface on the food contact side of the container or plate.
Microwaveable, disposable, rigid and strong containers and plates having suitable food contact compatible olfactory properties have been prepared. These disposable and microwaveable articles of manufacture exhibit (a) suitable food contact compatible olfactory properties; and (b) a melting point of not less than 250xc2x0 F., suitably 250xc2x0 F. to 330xc2x0 F. In preferred embodiments these articles of manufacture exhibit a micronodular surface on the side coming in contact with food. These microwaveable, food contact compatible containers and plates are dimensionally stable and resistant to grease, sugar and water at temperatures of at least 220xc2x0 F. and are of sufficient toughness to be resistant to cutting by serrated polystyrene flatware. The containers and plates of this invention answer a long felt need for products which can withstand the severe conditions of a microwave oven when common foods such as beans and pork, pancakes with syrup, pepperoni pizza, and broccoli with cheese are microwaved during food cooking and reconstituting processes.
It has been found in accordance with the present invention that polypropylene/mica food contact articles such as bowls or plates exhibit suitable olfactory characteristics when prepared by a low temperature process and/or when prepared including a basic organic or inorganic compound. There is provided in a first aspect of the present invention, a microwaveable, disposable food service article having food contact compatible olfactory properties formed of a melt processed polyolefin/mica composition wherein the composition includes from about 40 to about 90% by weight of a polypropylene polymer and from about 10 to about 50% by weight mica where the melt processed composition exhibits low odor as characterized by a relative aroma intensity index of less than about 1.6. Less than about 1.5 is more preferred and, as a practical matter, the lower limit of the relative aroma intensity index for the inventive composition is believed to be about 0.1.
Typically, the melt processed composition from which the microwaveable article is formed also includes a basic organic or inorganic compound including the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides including mixtures of silicone dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of the foregoing. More specifically, the basic organic or inorganic compound may be selected from the group consisting of: calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium citrate, potassium citrate, calcium stearate, potassium stearate, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicone dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of one or more of the above. Furthermore, hydroxides of the metals and alkaline earth elements recited above may be utilized.
Where a basic inorganic odor suppressing compound is chosen, generally such compound is selected from the group consisting of calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicone dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of one or more of the basic inorganic compounds set forth above. The amount of a basic inorganic compound is generally from about 2 to 20 weight percent, but is usually from about 5 to about 15 weight percent of the article. Most preferably the basic inorganic compound selected is calcium carbonate; typically present from about 5 to about 20 weight percent.
Where an organic compound is chosen, it is typically selected from the group consisting of sodium stearate, calcium stearate, potassium stearate, sodium citrate, potassium citrate, and mixtures of these where the amount of such compound is from about 0.5 to about 2.5 weight percent of the article.
Typically, microwaveable articles produced in accordance with the present invention exhibit a relative aroma intensity index of less than about 1.0; preferably less than about 0.7; with a practical lower limit being 0.1 or so.
As shown below in connection with microwaveability testing, and summarized in Table 20, competing commercial polystyrene type plates cannot withstand the high temperatures generated in the microwave oven during food contact and either significantly warp or deform when the aforementioned food products were heated on them. Under the usual microwaving conditions with high grease content foods, the prior art plates tend to deform and flow to the point where parts of the plate become adhered to the inside of the microwave oven. For disposable plates and containers, having suitable food contact olfactory properties, appearance and feel are important attributes. The micronodular surface of the plates and containers of this invention where mica and the basic inorganic compound or basic organic compound are used in combination with polypropylene or polypropylene polyethylene copolymers or blends tend to give these products the pleasing appearance, feel of stoneware or a pottery-like look and suitable food contact olfactory properties. Another significant property of the containers and plates of this invention is their cut resistance. These rigid articles of manufacture are of sufficient toughness to be resistant to cutting by serrated polystyrene flatware. In normal usage they are also resistant to cutting by regular metal flatware.
Whereas any microwaveable article may be produced in accordance with the invention, most typically the article is a bowl or a plate suitable for serving food at a meal. The articles may be produced by injection molding; however, preferred articles are thermoformed and include a micronodular food contact surface. Micronodular food contact surfaces are produced by thermoforming a sheet into the article which has been extruded optionally with at least one matte roll and by vacuum thermoforming the sheet by applying vacuum opposite to the surface where the micronodular surface is desired. Most typically the micronodular surface will have a surface gloss of less than about 35 at 75xc2x0 as measured by TAPPI method T-480-OM 92. Articles also will typically have a Parker Roughness Value of at least about 12 microns.
While any suitable polypropylene polymer may be used, the polypropylene polymers are preferably selected from the group consisting of isotactic polypropylene, and copolymers of propylene and ethylene wherein the ethylene moiety is less than about 10% of the units making up the polymer, and mixtures thereof. Generally, such polymers have a melt flow index from about 0.3 to about 4, but most preferably the polymer is isotactic polypropylene with a melt-flow index of about 1.5. In particularly preferred embodiments, the melt compounded composition from which the resultant extruded sheet is formed into articles further includes a polyethylene component and titanium dioxide. The polyethylene component may be any suitable polyethylene such as HDPE, LDPE, MDPE, LLDPE or mixtures thereof.
The various polyethylene polymers referred to herein are described at length in the Encyclopedia of Polymer Science and Engineering (2d Ed.), Vol. 6; pp: 383-522, Wiley 1986; the disclosure of which is incorporated herein by reference. HDPE refers to high density polyethylene which is substantially linear and has a density of generally greater that 0.94 up to about 0.97 g/cc. LDPE refers to low density polyethylene which is characterized by relatively long chain branching and a density of about 0.912 to about 0.925 g/cc. LLDPE or linear low density polyethylene is characterized by short chain branching and a density of from about 0.92 to about 0.94 g/cc. Finally, intermediate density polyethylene (MDPE) is characterized by relatively low branching and a density of from about 0.925 to about 0.94 g/cc. Unless otherwise indicated these terms have the above meaning throughout the description which follows.
The microwaveable articles according to the invention typically exhibit melting points from about 250 to about 330xc2x0 F. and include mica in amounts from about 20 to about 35 weight percent. Most preferably mica is present at about 30 weight percent.
It has been found that C8 and C9 organic ketones correlate well with or are associated with undesirable odors in polypropylene/mica compositions. Accordingly, it is preferred that articles in accordance with the invention are substantially free from volatile C8 and C9 organic ketones. In order to avoid undesirable odors, articles in accordance with the invention are preferably prepared from a melt compounded polyolefin mica composition which is prepared at a process melt temperature of less than about 425xc2x0 F.; with below about 400xc2x0 F. being even more preferred. Optionally, the melt processed polyolefin/mica composition is melt compounded in a nitrogen atmosphere.
In another aspect of the invention there is provided a microwaveable, disposable food contact article having food contact compatible olfactory properties formed of a melt processed polyolefin/mica composition wherein said composition includes from about 40 to about 90 percent by weight of a polypropylene polymer and from about 10 to about 50 percent by weight mica and a basic organic or inorganic odor suppressing compound including the reaction product of an alkali metal or an alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides and silicates and basic metal oxides, including mixtures of silicone dioxides with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures thereof.
Preferably the inventive articles are prepared from a melt compounded polyolefin/mica composition prepared by way of a low temperature compounding process.
A preferred low temperature compounding process used for producing polypropylene/mica melt compounded compositions including a basic odor suppressing agent having olfactory properties suitable for food contact applications in accordance with the invention includes the sequential steps of: (a) preheating a polypropylene polymer while maintaining the polymer below a maximum temperature of about 350xc2x0 F. and more preferably below a maximum of about 260xc2x0 F.; but suitably above about 240xc2x0 F.; followed by; (b) admixing mica to said preheated polymer in an amount from about 10 to about 50 percent weight based on the combined weight of the resin and mica and maintaining the mixture below about 425xc2x0 F.; followed by, (c) extruding the mixture. Polymer may be melted exclusively through the application of shear, or the shear may be supplemented through heating by infrared radiation or ordinary heating coils or performed externally to the mixing chamber. Preferably, the basic odor suppressing agent is added simultaneously with the mica.
It is desirable to keep the duration of the step of admixing mica and a basic odor suppressant agent to the mixture relatively short so as not to generate compounds which cause odor and to preserve the particle size and aspect ratio of the mica. Accordingly, the step of admixing the mica should be no more than about five minutes with the duration of the admixing step of less than about three minutes being even more preferred. Any suitable means may be used to carry out the sequential process in accordance with the invention, however, the process is normally carried out in a batch mode in a mixing chamber provided with a pair of rotating rotors in an apparatus referred to in the industry as a Banbury type mixer. One may choose to use a twin screw extruder or a Buss kneader to practice the inventive process if so desired, provided that appropriate elements are used to minimize shear heating.
In a further aspect of the invention, there is provided a process for making pottery-like, micronodular, low-odor microwaveable containers. The inventive process is generally directed to a process for forming a microwaveable, disposable, rigid and strong, mica and basic inorganic or organic compound filled polyolefin containers having food contact compatible olfactory properties, the polyolefin being selected from the group consisting of polypropylene and polypropylene polyethylene copolymer or blend, and a mixture of these wherein the inorganic or organic compound is selected from the group consisting of calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium phosphate, potassium phosphate, magnesium phosphate, sodium stearate, calcium stearate, potassium stearate, sodium citrate, potassium citrate, hydroxides of these elements, and mixtures of these organic compounds, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of one or more of the basic inorganic or organic compounds set forth herein. The process involves the steps of:
(a) forming an extrudable admixture of the polyolefin resin, mica, and the basic inorganic compound or basic organic compound;
(b) extruding the extrudable admixture of the polyolefin resin, mica, and the basic inorganic compound or the basic organic compound at elevated temperature;
(c) passing the resulting extruded admixture of the polyolefin resin and mica and the basic inorganic compound or the basic organic compound through a multiple roll stack, at least one roll of said stack having a matte finish;
(d) thermoforming the extruded admixture of the polyolefin, resin, mica, and the basic inorganic compound or organic compound; and
(e) recovering a container having a micronodular surface and exhibiting a melting point of no less than 250xc2x0 F.
The container is dimensionally stable and resistant to grease, sugar, and water at temperatures up to about 220xc2x0 F. and has sufficient toughness to be resistant to cutting by serrated flatware. The amount of the basic inorganic compound or basic organic compound added is sufficient to reduce carbonyl moiety containing decomposition products to provide containers with suitable food contact compatible olfactory properties.
The process most preferably includes:
(a) forming an extrudable admixture of the polyolefin resin, mica, and the basic inorganic compound or basic organic compound;
(b) extruding the extrudable admixture of the polyolelfin resin and mica and the basic inorganic compound or the basic organic compound at elevated temperature;
(c) passing the resulting extruded admixture of the polyolefin resin and mica and the basic inorganic compound or the basic organic compound through a multiple roll stack, at least one roll of the stack having a matte finish;
(d) passing the extruded admixture of the polyolefin resin, mica, and basic inorganic compound or the basic organic compound at least partially around the roll having a matte finish;
(e) controlling the speed of the extrusion process, the size, temperature and configuration of the roll stack such that the surface of the extruded admixture of the polyolefin resin, mica, and the basic inorganic or organic compound not in contact with the matte roll has a coarse-grained structure;
(f) thermoforming the extruded admixture of the polyolefin, resin, mica, and the basic inorganic compound or organic compound; and
(g) recovering a container having a micronodular surface and a rough surface and exhibiting a melting point of no less than 250xc2x0 F.
The coarse-grained structure of the surface of the extruded admixture of the polyolefin resin, mica, and the basic inorganic compound or basic organic compound not in contact with said matte roll is formed by transversing the extruded admixture of the polyolefin resin, mica, and the basic inorganic compound or basic organic compound through a curvilinear path and at least partially solidifying the surface of the extruded admixture of polyolefin resin, mica, and the basic inorganic compound or basic organic compound not contacting said matte roll while that surface is in tension relative to the surface contacting said matte roll. The container may be a plate, a cup, a bowl, a tray, a bucket, a soufflxc3xa9 dish or the like.
Thermoforming is typically conducted at a sheet temperature of from about 260xc2x0 to about 310xc2x0 F., and more preferably at a temperature of from about 280xc2x0 to about 300xc2x0 F.
There is provided in a still further aspect of the invention a crack-resistant, thermoformed food contact article having a wall thickness ranging from about 10 to about 80 mils consisting essentially of from about 40 to about 90 weight percent of a polypropylene polymer, from about 10 to about 50 percent by weight mica, from about 1 to about 15 percent by weight polyethylene, from about 0.1 to about 5 weight percent titanium dioxide and optionally including a basic organic or inorganic compound. The basic compound is, generally speaking, the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides, including mixtures of silicone dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures thereof. A particularly preferred article is where the basic organic or inorganic compound is calcium carbonate which is present in an amount of from about 5 to about 20 weight percent.
Polyethylene is more typically present from about 2.5 to about 15 weight percent, preferably from about 4 to about 5 weight percent of the crack resistant article.
Titanium dioxide is included in various amounts, from about 0.1 to about 3 percent by weight being typical; from about 0.25 to 2 percent titanium dioxide may be included. Preferably, titanium dioxide is included in at least 0.5 percent by weight.
The caliper, or wall thickness, of the articles is usually from about 0.010 to about 0.050 inches or from about 10 mils to 50 mils. A caliper of from about 15 to 25 mils is most typically employed.
While any suitable polypropylene polymer may be employed, the most preferred polymer is isotactic polypropylene having a melt index in the range of from about 0.3 to 4, with a melt index of about 1.5 being typical. The polyethylene employed may be HDPE, LLDPE, LDPE or MDPE, mixtures thereof or a polyethylene with bimodal molecular weight distribution. Polypropylene is sometimes referred to hereafter as xe2x80x9cPPxe2x80x9d.
The inventive compositions from which the crack resistant articles are made do not include coupling agents such as maleic anhydride containing polypropylene as further described herein, but may optionally include other components which do not alter the basic and novel characteristics of the crack-resistant plates. For example, nucleants such as sodium benzoate in amounts detrimental to crack resistance are to be avoided.